Navigating Constitution

By Commander Tyrone G. Martin, U. S. Navy (Retired)

“Navigating” is locating oneself accurately and guiding oneself efficiently and safely from that location to another. It’s easy on land because there are so many natural and man-made reference points to use. On the high seas, there are none so permanent and obvious. How, then, have ages of mariners managed to do it? How was it done in “Old Ironsides”? Let us consider the tools available and how they were used.

The oldest navigating “instrument” on board was the leadline. As its name clearly states, it was a length of light rope with a lead weight attached to one end and its length marked off in “fathoms” (units of six feet). In a hollow in the bottom of the lead weight was stuffed a wad of wax. When in coastal waters, a seaman would stand on one of the platforms outside the ship used to spread the supporting rigging of the foremast (called the “fore chains”) and cast the lead. When he felt it hit bottom, he would notice which fathom mark was next above the surface and would call out his finding so the controlling officer back near the ship’s wheel could hear his report. (“By the mark twain” meant exactly two fathoms of water.) After pulling the lead up, the leadsman could then check the wax and from the particles stuck in it report the kind of bottom below them — yellow or gray mud, gravel, sand, etc.

A sailing ship always carried several of these “hand leads” for use in unfamiliar coastal waters or when fog or night prevented reference to known objects on shore. In addition, she would carry a deep sea (or “dipsey”) lead for use in still deeper waters. Because it took so long to reach deeper bottoms, and because it might be subjected to different currents at different depths, a dipsey lead was harder to use and less accurate. Even so, it might provide a ship with it first notice of nearing land after a transoceanic voyage at a time when the Sailing Master (navigator) believed they might be near and poor visibility prevented an expected sighting. It could prevent a grounding and loss of all hands.

The magnetic compass, by which means a ship is kept heading in a desired direction, seems to have come to Europeans via Arab sailors from the Chinese as a result of the Crusades (13th Century). By the early 19th Century, the compass had become a magnetized needle above a card showing the directional points around a circle (a “compass rose”), floating in a liquid to deaden oscillations. The compass itself was mounted in “gimbals” so that it could remain more constantly horizontal in a rocking and rolling ship. The two carried in Constitution were mounted in stands (“binnacles”) just ahead of the ship’s wheel where they could be seen by the helmsmen who steered her.

Charts – maps of the sea and shore – had been increasing in accuracy and detail since the Age of Discovery. In addition to depicting the shapes and features of shorelines, they provided information on water depths, currents, and the presence of known underwater dangers (reef,etc.). Overlaid on these “pictures” was a grid system by which a position could be located with or without fixed geographic reference points. The concept of the grid was originated by a Greek, Ptolemy, in the 4th Century, and in the 16th Century had been adapted to a form that greatly simplified its use for navigation by the Dutchman, Gerardus Mercator. (His “projection” is the one most frequently used for general purpose maps and charts today). The grid lines parallel to the one around the middle of the Earth (the “Equator”) were used to measure “Latitude” in degrees, minutes, and seconds of arc through 90 degrees north or south of the Equator (i. e., to the Poles). The grid lines perpendicular to them were similarly measured east or west of a “prime meridian” to 180 degrees (halfway around the globe).

Until the 16th Century, seamen estimated their ship’s speed through the water by noting the time it took a wood chip, or bubble, or piece of seaweed to pass along the length of their vessel and converting that distance and interval to velocity. Some time prior to 1578, the “log line” or “chip log” was invented to provide a more accurate measurement. It consisted of a triangular “chip” of wood attached to a light line by its three corners so that, when tossed overboard it “dug in” and pulled the line from its hand-held reel. Knots were tied in the line at intervals that equated to sea miles-per-hour. A half-minute sand glass was used as a timer. Once an hour in Constitution, the Midshipman and Quartermaster of the Watch, together with a seaman helper, went aft to the taffrail and tossed the log. When the chip hit the water, the Quartermaster turned the glass. At his “Mark,” the seaman would stop the run-out and as he reeled the line back in, the Midshipman noted how many knots had run out and any fraction of distance between the reel and the last knot to run. This sea-speed, in “knots,” was recorded on a slate kept near the ship’s wheel for the purpose, together with a record of every change of direction (“course”) and the time of change.

The final navigation instrument available to Constitution when she first went to sea in 1798 was the quadrant or sextant. The former had appeared in its latest form in 1731 and the more efficient sextant three decades later. With these instruments, it was possible to measure with precision the altitudes (angular heights) of celestial bodies. Their principal use was in measuring the altitude of the sun at local high noon, so that, with a minimum of mathematical calculation, the ship’s latitude could be accurately known. They could also be used to take lunar measurements that could be used to determine longitude, but the calculations necessary for that determination were so complex that few mariners had mastered them.

These, then, were the tools available to Constitution’s Sailing Master for locating his ship’s position and guiding her safely from place to place. What was his routine?

When the ship received her orders, the Sailing Master would mark his recommended track on a chart and confer with the Captain. On a short voyage through open water, the track would be a single course from “A” to “B.” An ocean crossing normally would be done as a “great circle sailing,” a series of course legs that together formed an arc of an imaginary circle around the Earth whose plane passed through the Earth’s center. Geometrically, it meant the shortest distance between points on the surface of a globe.

The Sailing Master took his “departure” as the ship sailed from port; that is, he established the reference point from which all his later estimates of the ship’s position would be measured. Perhaps it was a prominent rock near the harbor entrance, or maybe a lighthouse. At this time, the Officer of the Deck, the senior man controlling the ship’s movements at most times of the day or night, would begin keeping a record of course steered, times when course was changed, and, of course, the hourly chip log reading.

Let us assume the ship sailed early in the morning and that it was a sunny day. Shortly before calculated high noon, the Sailing Master and a Master’s Mate would appear, quadrants or sextants in hand, together with a Quartermaster to assist. The Master and his Mate each would take successive “shots” of the Sun until it reached its peak in the sky (“zenith”). At that instant, the Master would call “Mark,” both would then note the altitude measured, and the Quartermaster would turn the ship’s half-hour glass, thereby “resetting” everyone to the correct local time of noon. (Until 1846, the U. S. Navy’s day at sea officially went from noon to noon, bearing the date of the end of the period.)

Master and Mate would go below, compare their readings, make the necessary mathematical corrections for things like known mechanical errors in their instruments, and plot the resultant latitude value as a line on the appropriate chart. To determine where the ship was along that latitude, the Sailing Master would use the course and speed information from the slate and laboriously plot it out on the chart from his “departure.” Where the vertical line drawn through the resultant endpoint crossed the sun’s latitude line became the ship’s 12 o’clock position by “deduced reckoning” (often written as “ded. reckoning” and corrupted to “dead reckoning,” a term still in use today.) This would be reported to the Captain in writing, and he and the Master would confer on any course change to be made as a result of it.

This routine was followed day after day. On cloudy or stormy days, the Master was limited to plotting out the courses and distances run from the last noon position. If several days went by without a “sun line,” accurate knowledge of the ship’s position would be severely degraded. This was when knowledge accumulated by the Master and/or Captain on previous voyages in a particular sea became important, because it might be that the presence or absence of a current, or the color of the water, might be important clues as to just where the ship was. If it was thought that land could be in the offing, perhaps the dipsey lead would be used to get early notice of a rising bottom. Too, a cautious Captain might choose to lay to at night or sail through the hours of darkness on course calculated to keep him clear of land until he could see again. With land in sight, the Sailing Master could, if necessary, consult the descriptions and sketches in the appropriate volume of “sailing directions” to identify his landfall. This done, he would then advise the Captain regarding the direction of and distance to their destination.

In 1802, Nathaniel Bowditch of Salem, Massachusetts, first published his “American Practical Navigator,” a volume that simplified and revolutionized marine navigation. Among other things, it brought the complicated calculations associated with lunar observations and the calculation of longitude into the “real world” of the poorly educated mariner.

The major problem with determining longitude accurately had, for centuries, been in the inability to know the correct time. A ship’s motion disturbed the regular flow of sand in an hour glass; it rendered the crude mechanical timepieces of those days useless. Early in the 18th Century, both the British and French governments offered large prizes for the development of marine equipment capable of the required accuracy for determining longitude. The brothers John and William Harrison in 1765 made the “chronometer,” a timepiece of remarkable accuracy that soon was shown to be largely impervious to the hectic motions of a ship at sea. Solution of this important problem was at hand, but decades passed before many ships had expensive chronometers on board and before sea officers, either self-taught or through “on-the-job” training, learned Bowditch’s more efficient method of making the necessary calculations. (Constitution received her first chronometer in September or October 1812. None of her subsequent wartime logs (1812-1815) indicate that it was used for anything more than more precisely timed entries when recording events in the ship’s logbook.)

In the latter half of the 20th century, mariners can navigate by a whole range of electronic marvels including satellites orbiting Earth especially for that purpose. Some of these systems are so sophisticated that the young Navigator has merely to turn a receiver on and read the positional values directly from an LED readout, then plot them on his chart. Getting him to learn to use the sextant and make the necessary calculations, and then to use the skill periodically so that it remains available, is becoming increasingly difficult. But it is a fact that electronic gizmos sometimes fail, while the sun, moon, planets, and stars go on and on and on.